1. Field of the Invention
The present invention relates to the field of transparent conducting oxide (TCO) and solid-state Gallium Nitride-based light-emitting devices.
2. Description of the Related Art
In recent years, the p-type Gallium Nitride-based III-V group compound semiconductor layer usually has a carrier concentration of less than 1×1018 cm−3, and the lowest resistivity is no lower than 1 ohm-cm. Such poor conductivity cannot effectively distribute the current to the entire p-type compound semiconductor layer. Therefore, the p-electrode is usually formed to cover substantially entire surface of the p-type Gallium Nitride-based III-V group compound semiconductor layer in order to ensure the uniform spreading of current to the entire p-type compound semiconductor layer, thereby obtaining uniform light emission from the device. However, the p-electrode considered being a light transmitting and ohmic electrode.
Due to the end of 1993, the Nichia Chemical Industries Ltd. in Japan announced the successful fabrication of the solid-state Gallium Nitride-based light-emitting devices that employed the metallic thin film as the p-electrode. Also, Nichia Chemical Industries Ltd. in Japan announces a particularly preferable metallic thin film contains gold and nickel. Gold and nickel are preferable formed such that a layer of nickel is formed in direct contact with the p-semiconductor layer and a layer of gold is formed on the nickel layer. After an annealing treatment, such a multi-layered structure can form an alloy, which is light transmitting and ohmic to p-type Gallium Nitride-based III-V group compound semiconductor layer.
The prior art shown in FIG. 1, the Nichia Chemical Industries Ltd disclosed in U.S. Pat. No. 6,093,965 that the Gallium Nitride-based III-V group compound semiconductor uses a sapphire substrate 116, an n-type Gallium Nitride-based cladding layer 15, an n-type Titanium/Aluminum (Ti/Al) electrode bonding pad 14, an Indium-Gallium Nitride system light emitting layer 13, a p-type Gallium Nitride-based cladding layer 12, a p-type metallic thin film contains Nickel-Gold (Ni/Au) light transmitting electrode 11A and a p-type Nickel-Gold (Ni/Au) electrode bonding pad 10.
A metallic thin film contains Nickel-Gold (Ni/Au) light transmitting electrode usually transmits 20 to 40% of the light emitted from device there through. Therefore, to improve the brightness and efficiency of the Gallium Nitride-based III-V group compound semiconductor light emitting device is to reduce the absorption from the light transmitting electrode.
Tin indium oxide has been used to be the light transmitting electrode to reduce the absorption from regular nickel-gold thin film light transmitting electrode as shown in FIG. 2, the Indium Gallium Nitride light emitting diode uses a sapphire substrate 116, an n-type Gallium Nitride-based cladding layer 15, an n-type Aluminum Gallium Nitride-based cladding layer 15A, an n-type Titanium/Aluminum (Ti/Al) electrode bonding pad 14, an Indium-Gallium Nitride system light emitting layer 13, a p-type Gallium Nitride-based cladding layer 12, a p-type high concentration contact layer 117, an Indium-Tin Oxide (ITO) light transmitting electrode 11C and a p-type Nickel-Gold (Ni/Au) electrode bonding pad 10.
A Gallium Nitride-based contact layer with a p-type concentration of greater than 5×1018 cm−3 and a thickness of less than 500 Angstroms. The contact layer 117 can be formed by Zinc (Zn) diffusion, Magnesium (Mg) diffusion, Zn or Mg ion implantation, etc, and the Epistar Co. uses Indium-Tin Oxide (ITO) to be the light transmitting electrode to improve the light efficiency. Usually, the light emitting diode of using this technique can only transmits 50 to 70% of the light emitted from light through the high concentration of p-type contact layer 117 by Zn, Mg diffusion or implantation process and ITO light transmitting electrode 11C. Furthermore, In ITO film, charge carriers are from both Tin dopant and ionized oxygen vacancy donors. The humidity can easy diffuse into ITO film and destroy the interface between ITO film and Gallium Nitride-based contact layer; the contact resistivity of ohmic contact will increase a lot. So, it is unstable and unreliable in high humidity condition.
In the prior art, no light transmitting electrode has the light efficiency and good reliability at the same time for Gallium Nitride-based light emitting semiconductor device.